The Titan IIIE was the largest and most powerful US launch vehicle yet developed for robotic exploration of the Solar System. While NASA had considered using the Titan IIIC and Transtage, development problems with Transtage led to the decision to integrate Centaur with Titan. However, Centaur had been designed for the very different Atlas vehicle and a number of changes were needed to accommodate it to Titan, the biggest being encasing the stage in a large shroud that bulged outward because of the different diameter of the two. There was also concern about the heat generated by the Titan's large solid boosters and hypergolic engines, requiring design of an insulation system that would protect Centaur's cryogenic propellants without adding unnecessary weight. The Centaur also contained the guidance computer for the entire launch vehicle. A four-stage configuration, with an additional upper stage, a Star-37E, was also available, and was used for the two Helios launches.[2] Star-37E stages were also used on the two Voyager launches, but were considered to be part of the payload rather than the rocket.[3]

The first Titan IIIE launch occurred on February 11, 1974. Original plans were to fly a boilerplate Viking probe, but NASA decided to add a secondary payload: a test satellite called SPHINX (Space Plasma High Voltage Interaction Experiment) which was intended to test the operation of high voltage power supplies in the vacuum of space. The mission was unsuccessful; while the Titan booster performed normally, the Centaur's engines failed to start. Ground controllers waited and issued a manual start command, but still nothing happened. 12 minutes after liftoff, the range safety destruct command was sent from a radar station in Antigua. The failure was traced to the Centaur boost pumps, but the cause was still unclear, thought likely to be either ice or debris. To reduce the chance of a recurrence of the failure, pre-launch procedures to verify Centaur's pumps were free and unobstructed were put in place. It took nearly four years to trace the cause of the failure to an improperly installed clip inside the LOX tank, which came loose and lodged in one of the boost pumps.[4] Despite the failure, at least one important goal was accomplished in that the bulging Centaur payload shroud was proven to be aerodynamically stable in flight and had jettisoned properly and on schedule. All subsequent launches were successful.